remove unused variable from DropdownActions class

src/Grid/Displayers/DropdownActions.php

@@ -10,11 +10,6 @@ use Dcat\Admin\Support\Helper;
 
 class DropdownActions extends Actions
 {
-    /**
-     * @var array
-     */
-    protected $custom = [];
-
     /**
      * @var array
      */